This section provides background information related to the present disclosure which is not necessarily prior art.
It is known that a conventional fuel injection system for a combustion engine includes a fuel rail and a plurality of electrically controlled fuel injectors, which are hydraulically connected with the fuel rail by respective feeding conduits.
Each fuel injector generally includes a fuel inlet, a fuel outlet and a movable needle, which repeatedly opens and closes the fuel outlet. When the needle is in an open position, fuel is injected under pressure into a cylinder of the engine. The movable needle is actuated with the aid of a dedicated actuator, typically a solenoid actuator or a piezoelectric actuator, which is driven by an electric circuit controlled by an engine control system (ECS). The ECS operates each injection pulse by generating control signals acting upon a control valve and causing the needle to open the fuel injector.
In order to improve the characteristics of exhaust emissions and reduce combustion noise in engines, particularly in Diesel engines having a common-rail fuel injection system, so-called multiple fuel injection patterns are adopted. In a multi-injection pattern, the fuel quantity to be injected in each cylinder at each engine cycle is divided into a plurality of injections. More specifically, in a multi-injection pattern, for each engine cycle, a series or train of injections is performed by each injector, typically starting from a pilot injection and following with a main injection, which gives all or most of the torque in an engine cycle, eventually terminating with post injections.
The number of injections in the train of injections and their timing is dependent on the combustion mode and is determined by the ECS. A pilot injection before a main injection strategy is able to give benefits in terms of brake specific fuel consumption (BSFC) and/or combustion noise (CN) and/or soot emissions, depending on the calibration used. In particular, a pilot injection before a main injection is an enabler for a better fuel spray atomization and therefore increases combustion efficiency.
Zero hydraulic interval (ZHI) between a pilot injection and a main injection, namely a condition in which there is no interval between the hydraulic closing of the needle of the injection after a pilot injection and the hydraulic opening of the needle for a main injection has previously been targeted. However, ZHI is a critical and difficult condition to reach and maintain because the electrical dwell time (DT) range for controlling the ZHI is very narrow and therefore it is difficult to maintain it along engine life due to disturbances such as injectors aging drift.
Future engine, particularly diesel engine, legislations will require more stringent targets for emissions and fuel consumption. As discussed above, multi-injection patterns allow an optimal trade-off between NOx and soot emissions at a certain combustion efficiency. Strategies have been investigated with very close injection pulses in order to further improve combustion noise and fuel efficiency whilst avoiding increased NOx emissions.
Injector ageing can be problematic when very close injection patterns are used. Very small hydraulic dwell time strategies are sensitive to pressure wave propagation phenomena and needle/servo-valve dynamics that could affect significantly the hydraulic injection rate, injection stability and injection deviation behavior over the life of the injector.
Accordingly, it is desirable to provide methods and systems that are able to control the injectors in a way that reduces or eliminates at least some of the negative effects of injector ageing. In addition, it is desirable to reduce or avoid changes in hydraulic injection rates caused by injector ageing and very close injection multiple injection strategies. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.